The present invention relates to resistive thin films, particularly to metal-oxide thin film resistors, and more particularly to Tixe2x80x94Crxe2x80x94Alxe2x80x94O thin film resistors and a process for fabricating same.
The development of metal-oxide materials has been widely pursued in the electronics industry for use as resistive thin films. The use of multiple phases provides a path to change the film resistivity. See C. A. Neugebauer, xe2x80x9cResistivity of Cermet Films Containing Oxides Of Siliconxe2x80x9d, Thin Solid Films, 6 (1970), 443-447. The dependence of sheet resistivity on composition is well established for systems such as Crxe2x80x94Sixe2x80x94O. See. R. Glang et al., xe2x80x9cResistivity and Structure of Crxe2x80x94SiO Cermet Filmsxe2x80x9d, J. Vac. Sci. Technol., 4 (1967), 163-70; A. A. Milgram et al., xe2x80x9cElectrical and Structural Properties of Mixed Chromium and Silicon Monoxide Filmsxe2x80x9d, J. Appl. Phys. 39 (1968), 4219-4224; N. C. Miller et al., xe2x80x9cCo-sputtered Cermet Filmsxe2x80x9d, Solid State Tech. 11 (1968), 28-30; and H. Steemers et al., xe2x80x9cStable Thin Film Resistors For Amorphous Silicon Integrated Circuitsxe2x80x9d, Mat. Res. Soc. Symp. Proc., 118 (1988), 445-449. The conduction mechanism for these cermet materials (materials composed of ceramics and metals) can be considered quantum mechanical. See. J. E. Morris, xe2x80x9cStructure and Electrical Properties of Auxe2x80x94SiO Thin Film Cermetsxe2x80x9d, Thin Solid Films, 11 (1972), 299-311. For low metallic concentrations, the charge transport is proposed to be by electron tunneling between the metallic particles. See B. E. Springett, xe2x80x9cConductivity Of A System Of Metallic Particles Dispersed In An Insulating Mediumxe2x80x9d, J. Appl. Phys., 44 (1973), 2925-2926. In general, conduction may be considered to be by means of an activated charge transport process. For film resistivities  greater than 10xe2x88x922 Ohm-cm, the microstructure is usually comprised of a continuous insulating matrix in which metallic particles are dispersed. An increase in metallic content produces a decrease ion sheet resistivity. For the Crxe2x80x94Sixe2x80x94O system, the insulating matrix is based on the oxide phase of SiO2, with Cr, silicides, and monoxides serving as conductors/semiconductors. A general observation by Neugebauer, Supra, suggests that the SiO2 composition alone could be used to determine the cermet film resistivity to within two orders of magnitude irrespective of deposition technique or conditions. Whereas this summation may represent a general trend, it is not an inclusive statement for the resistivity behavior of Crxe2x80x94Sixe2x80x94O cermets. Initial work at the Lawrence Livermore National Laboratory with the Crxe2x80x94Sixe2x80x94O cermet system has shown a widely varying range of resistivities that span more than twelve-orders of magnitude and are often accompanied by a non-linear current-voltage behavior. See A. Jankowski et al., xe2x80x9cResistivity Behavior Of Crxe2x80x94Sixe2x80x94O Thin Filmsxe2x80x9d, Chem. Phys. Nanostructures and Related Non-Equilibrium Materials, ed. E. Ma. et al., The Minerals, Metals and Materials Soc. Proc. (1997), pg. 211-219. In addition, post-deposition vacuum annealing can cause changes in the resistivity by several orders of magnitude rendering unreliable use of the Crxe2x80x94Sixe2x80x94O film as a resistor layer of constant value. Due to the limitations of producing a consistent resistivity from 105 to 108 Ohm-cm for the Crxe2x80x94Sixe2x80x94O system, an alternate material has been sought which would have a well-defined and stable behavior as a resistor layer.
The present invention provides the sought for alternate for the Crxe2x80x94Sixe2x80x94O system, and it has been determined that the system of the present invention has a well-defined and stable behavior as a resistor layer. The Tixe2x80x94Crxe2x80x94Alxe2x80x94O cermet of the present invention is being developed for use as a thin film resistor since its properties in bulk form are favorable and controllable. The Tixe2x80x94Crxe2x80x94Alxe2x80x94O films are radio frequency (rf) sputter deposited to transfer the target composition to the growing cermet film. The films are rf sputter deposited from ceramic targets using a reactive working gas mixture of Ar and O2. The film resistivity can be discretely selected through target composition and the control of the deposition parameters.
It is an object of the present invention to provide metal-oxide resistive thin films which have a well-defined and stable behavior.
A further object of the invention is to provide a metal-oxide thin film which is thermodynamically stable.
A further object of the invention is to provide Tixe2x80x94Crxe2x80x94Alxe2x80x94O thin film resistors.
Another object of the invention is to provide a Tixe2x80x94Crxe2x80x94Alxe2x80x94O cermet which can be effectively utilized as a resistor material.
Another object of the invention is to provide a process for fabricating Tixe2x80x94Crxe2x80x94Alxe2x80x94O thin film resistors.
Another object of the invention is to provide a process for producing Tixe2x80x94Crxe2x80x94Alxe2x80x94O ceramic targets and films by rf sputter deposition from the ceramic targets using a reactive working gas mixture of Ar and O2.
Another object of the invention is to provide a process for fabricating Tixe2x80x94Crxe2x80x94Alxe2x80x94O films wherein the resistivity of the film can be discretely selected through control of the deposition parameters.
Other objects and advantages of the invention will become apparent from the following description and accompanying drawings. The present invention is directed to Tixe2x80x94Crxe2x80x94Alxe2x80x94O cermets which can be utilized as a resistor material, and to a process for fabricating Tixe2x80x94Crxe2x80x94Alxe2x80x94O thin film resistors. The films are rf sputter deposited from ceramic targets using a reactive working gas mixture of Ar and O2, and having, for example, a ceramic powder blend of 2-12% TiO2, 30-40% Al2O3, and 50-65% Cr2O3, with a film composition, for example, of 1-3 at.% Ti, 15-20 at.% Cr, 10-20 at.% Al, and 58-70 at.% O. The films are deposited to a thickness  greater than 0.2 xcexcm in order to avoid effects often seen in metalxide films  less than 0.1 xcexcm thick. See T. Filutowicz et al., xe2x80x9cThe Effects Of Film Thickness On Certain Properties Of Crxe2x80x94SiO Cermet Thin Filmsxe2x80x9d, Electron Technology. 10 (1977), 117-126; and H. S. Hoffman et al., xe2x80x9cCermet Resistors On Ceramic Substratesxe2x80x9d, IEEE Trans. On Components. Hybrids And Manufacturing Technol., 4 (4) (1981), 387-395. The film resistivity can be discretely selected through control of the target composition and the sputter deposition parameters. The application of Tixe2x80x94Crxe2x80x94Alxe2x80x94O as a thin film resistor has been found to be thermodynamically stable, unlike other metal-oxide material systems.